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Choosing a battery charger
The purpose of rechargeable batteries is economy. Simple arithmetic on the
manufacturers estimate of 300 to 1000 charge cycles demonstrates the savings
over single use batteries.
These figures are based on ideal use and charging conditions created in manufacturers
tests. The closer you keep to those test conditions in use the longer the battery
life.
There three main factors affecting charger choice.
- Size and type of batteries (AA, AAA, C, D, PP3, CRV3 or special camera batteries)
- Charging speed (Ranges from 15 minutes up to 15 hours)
- Economics - faster charging costs more. (£4 to £40 per charger)
A rechargeable battery produces heat as it is charged, causing expansion and
a pressure build up that could damage it. Batteries designed for rapid charging
have vents to relieve the pressure. They could still be damaged by excessive
heat
Exceeding the battery's current rating in use, or it's current / temperature
limits when charging can reduce the useable cycles to 10 or less in extreme
circumstances. This is the worst case. However, it is very easy to reduce the
useful life by up to two thirds through a lack of care.
Low cost chargers deal with these potential problems by charging the
battery slowly. Leaving the battery on charge too long is not an issue, as they
do not charge fast enough to overheat it. However, the charge time can be up
to 15 hours. Automatic timers speed things up by charging faster and
automatically stop charging after a predetermined time.
Intelligent chargers test the battery before starting to charge it to
determine it's capacity and state of charge. The charge rate is varied
according to the heat generating characteristics of the battery. Some also monitor
the battery temperature. A technique known as 'Delta V cutoff'
detects the small voltage drop in a battery just as it reaches a full charge.
This allows charging to be stopped at the appropriate time. Intelligent chargers
dramatically shorten the charging time, provide longer run times, and prolong
the working life of a battery.
There are three strategies to rechargeable battery management
- Adopt a throwaway attitude. Buy low cost Ni-Cd batteries, a low
cost charger, and accept batteries will not last for 500 charge cycles.
- Understand the battery technology and treat it with tender loving care.
- Use an intelligent charger and let it work it out for you.
The above obviously affects charger choice. The faster you need to charge your
batteries without unnecessarily shortening their life, the higher specification
charger you will need.
Charger feature comparisons
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Feature
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Charger Type
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Functional Description
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Built-in IC (Integrated Circuit) timer
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Mid-range
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Charger switches off
automatically after a set time to avoid overcharging batteries.
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Automatic charge current selection
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Mid to high end
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Chargers with this capability work
out how many and what capacity of batteries have been installed without the
user having to remember to set the right switch
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Temperature sensor
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Intelligent
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Ensures batteries do not overheat
on fast charge.
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Overcharge protection or
Negative Delta V (-dV)
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Intelligent
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Detects when batteries are fully
charged so charging can be stopped. More accurate than timers.
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Faulty battery detection
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Intelligent
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Detects if batteries have a short
circuit or cannot take a charge. Will not charge if there is risk to the
batteries or charger.
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Short Circuit Protection
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Mid-range
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Protects charger from overload
caused by a short circuit battery.
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Reverse Polarity Detection
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Protects both batteries and
charger from loading batteries the incorrectly.
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Intelligent Microprocessor
controlled
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High end
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Varies charge current over the
charge cycle allowing batteries to be charged in the shortest time without
overheating
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Trickle charge
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High end
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Ensures batteries are maintained
at full charge until needed.
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